Passive wireless transponders are known in the art. “Passive” in this context means that the transponder includes no internal energy source, such as a battery. Typically, such transponders receive the energy they need to operate by induction from an external radio-frequency (RF) electromagnetic field. For this purpose, the transponder generally comprises both a power antenna, for receiving energy from the field, and a communication antenna, for transmitting and/or receiving communication signals to and/or from an external base station. Such transponders may be used, inter alia, to transmit and receive signals used in determining the location of an object within the body of a patient. Transponders of this sort are described, for example, in U.S. patent application Ser. No. 10/029,473, to Govari, filed Dec. 21, 2001, and published as U.S. 2003/0120150 A1, which is assigned to the assignee of the present patent application, and whose disclosure is incorporated herein by reference.
As another example, U.S. Pat. No. 6,239,724, to Doron et al., whose disclosure is incorporated herein by reference, describes a telemetry system for providing spatial positioning information from within a patient's body. The system includes an implantable telemetry unit having (a) a first transducer, for converting a power signal received from outside the body into electrical power for powering the telemetry unit; (b) a second transducer, for receiving a positioning field signal that is received from outside the body; and (c) a third transducer, for transmitting a locating signal to a site outside the body, in response to the positioning field signal.
PCT patent publication WO 00/38571 A1 and U.S. Pat. No. 6,261,247, to Ishikawa et al., whose disclosures are incorporated herein by reference, describe an anatomical position sensing system using one or more substantially spherical transponders for measuring relative positions and distances. The transponders are capable of receiving and transmitting RF signals, thus communicating between themselves and with a separate CPU. The CPU controls a broadband antenna to transmit a low-frequency RF power signal to energize the transponders. Once energized, the transponders transmit range signals in all directions at other frequencies. These signals are used in determining the positions of the transponders.
In one embodiment described by Ishikawa et al., the transponder is fabricated on a spherical substrate, and includes nine coils in three sets of three coils. Each set is orthogonal to the others and comprises three coils: one transmit coil, one receive coil, and one power coupling coil. The coil sets are grouped in this fashion to ensure that at least one coil set is oriented to provide potentially optimum power coupling and signal communication therewith. Each of the power coupling coils is connected to a power circuit, which rectifies the varying magnetic energy coupled into the coil. The power circuits are connected in series to provide power to the other transponder circuits.